This instructable covers how to etch a knife blade.
Modern knifes are often made from stainless steel and other corrosion resistant materials. To encourage etching electricity can be used. Don't worry, 9V is enough!

Etching is different from engraving. Engraving is usually scratching fine lines into the surface of the material, whereas etching chemically removes material, possibly until a hole is created! Etching can often go deeper than engraving.

I should mention that I got the information on how to do this from this site , it came up from a google result and gives you the basic info on how to do it. I've added pictures of my experience to help you along the way

Step 1: Materials

You're going to need:
-nail varnish (paint may do instead),
-acetone (nail varnish removed or other cleaning solvent might do),
-cotton buds,
-water,
-salt,
-a 9V battery,
-croc clips,
-a scribe or pin,
-the object to be engraved,
-a dish to do it in.

Step 2: Coat the Blade

I practiced on some cheap snap off blades and found that any uncoated material near the etching area became etched and pitted. So to be on the safe side I entirely painted the blade with nail varnish.

Step 3: While That's Drying...

Get the dish you intend to do this in (not your mum's best cookware, so scrap would be better), add the water and add some salt. You want to make salty water. I'm assuming the saltier the quicker the reaction. This might be an interesting experiment to carry out with varying concentrations of salt and measuring etch depth over a set time.
I just added a bit of salt, don't think I got anywhere near saturating the water, just enough to make it salty - a good electrolyte.

Step 4: Scribe the Pattern

Engraving is quite tricky and unforgiving.
Scribing a pattern in (relatively) soft nail varnish isn't so tough.
You don't need to press so hard you score the metal, just enough to remove the nail varnish in the pattern you want etching.
Its practically like normal writing.
If you go wrong you'll have to repaint and retry or even clean it all off and then repaint and retry.

This knife is a gift for someone who has organised our summer climbing holiday which also coincides as my honeymoon, I'm really grateful he's found us such a great place to go and organised everything so got him this knife as a thank you.

Venasque is the place we're visiting, and the year 2011.

Step 5: Electrification!!

Connect the object to be etched to the positive terminal of a 9V battery.
Apparently a PP3 square battery works fine, I didn't have any on hand but had this beefy old 12V lead acid cell. It worked great.
Connect the other end to the object to be etched. It needs a good electrical connection, don't connect it where you have painted the nail varnish, the hole point in the nail varnish is to insulate the blade and stop the electrical connection.

Step 6: Etch

Connect the cotton bud to the negative side of the battery (cotton buds aren't conductive so make sure to connect to the end you will be using).
Submerge the nail varnished and patterned part of the blade in the salt water and gently rub the cotton bud over the patterned area.
You should see lots of tiny bubbles as the electricity flows through the salt water and corrodes away the blade where you have marked your pattern in the nail varnish. Everything else on the blade should be safely coated in nail varnish and not react.

In my experience less is more. The patterns I hung about on in the water and tried to make deeper ended up messier and ill defined.

Don't reverse the polarity (negative to the blade, positive to the cotton bud). This produces more bubbles but no etching and appears to life the nail varnish which you so carefully scribed.